Bismuth is one of the most deleterious impurities affecting the quality of electrorefined copper. For the production of good quality copper acceptable by rod mills, the bismuth concentration should not exceed 0.2-0.3 ppm. However, with the depletion of cleaner copper orebodies around the globe, more and more smelters are now being forced to accept and treat dirtier feed materials including high concentrations of bismuth. The smelting of high bismuth containing feed materials results in most cases in the production of copper anodes having high bismuth concentrations, and puts pressure on the electrorefining operations.
During the electrorefining of copper anodes which is conventionally carried out in a sulfuric acid/copper sulfate solution, impurities either dissolve in the electrolyte, or precipitate in the slime layer. Bismuth, arsenic and antimony present in the anode report both to the slime and electrolyte phases in proportions which may to some extent depend on other impurities in the anodes as well as their respective concentrations. Once in the electrolyte, the bismuth can contaminate the cathode via different mechanisms. For example, if the electrolyte bismuth level is left to reach its saturation level which is only 0.3-0.4 g/L, bismuth oxides can precipitate and contaminate the copper deposit on the cathode. Also, bismuth contamination of the cathodes may occur by complex precipitates (float slimes) formed with other impurities in the electrolyte or simply by the occlusion of the electrolyte in the copper deposit. In conventional copper electrorefining process a major part of the bismuth is removed during an electrolytic purification process whereby a bleed of tankhouse electrolyte is treated in electrowinning cells. Here, after initial decopperization bismuth is deposited onto a cathode along with other impurities such as arsenic and antimony to form a sludge.
When the bismuth input to the refinery is high, the electrolytic purification which is limited to the amount of dissolved copper in the electrolyte cannot maintain the bismuth concentration at a comfortably low level. Higher bismuth levels result in cathodes contaminated with high levels of bismuth and renders the product unacceptable for copper rod plants.
To help remove additional quantities of bismuth from copper electrolyte, some refineries have developed ion exchange purification systems. For example, in the Tamano process, the ion exchange resin removes both antimony and bismuth from electrolyte. The stripping of the loaded resin is carried out with concentrated hydrochloric acid which is recovered by distillation. The recovered hydrochloric acid is returned to stripping while both the antimony and bismuth are recovered as a sludge and further treated. The high concentrations of chloride ions in the electrolyte promote the precipitation of a cuprous chloride (CuCl) layer on the anodes causing them to passivate. Another disadvantage of the system is the very cumbersome distillation process which requires expensive equipment. Due to these inconveniences some refineries reportedly neutralize the hydrochloric acid with lime. This, on the other hand requires a costly neutralization process and the treatment or the disposal of the bismuth and antimony containing sludge. In another development, crown ethers are used to selectively remove bismuth from tankhouse electrolyte. The advantage of this process is that the loaded particles can be stripped by sulfuric acid rather then hydrochloric acid to yield a strip solution with a relatively concentrated bismuth level (up to 10 g/L). The use of sulfuric acid eliminates the potential hazard of concentrated hydrochloric acid solution accidentally entering the tankhouse which would have a devastating effect onto the electrolysis itself. However, the conventional way of treating this eluent is to neutralize it and produce a bismuth containing sludge for disposal or sending it back to a smelter. In the latter case some of the bismuth will find its way back into the metal refined during the smelting operations.